Associate a property with a parse tree node. Useful with parse tree listeners that need to associate values with particular tree nodes, kind of like specifying a return value for the listener event method that visited a particular node. Example:

ParseTreeProperty<Integer> values = new ParseTreeProperty<Integer>();
values.put(tree, 36);
int x = values.get(tree);
values.removeFrom(tree);

You would make one decl (values here) in the listener and use lots of times in your event methods.